Question

A student is told that both energy and mass must be conserved in every chemical reaction. She measures the mass of Hydrochloric acid and a zinc strip separately. She then places the zinc strip into the acid and bubbles form as the zinc looks like it disappears. The combined mass afterward is less than the original.

Answer 1

Answer:

Explanation:

When we react Hydrochlorid Acid with zinc we have the following reaction:

2HCl(aq) + Zn(s) --> ZnCl2(aq) + H2(g)

The hydrogen gas formed is lost to the environment, so we can affirme that in the start we have the mass for all the H, Cl and Zn atoms in the solution, but after the reaction occurs, we have only the mass for the Cl and Zn atoms.

That's why the mass is less than the original.

The law that the student was told is only applied to closed environments.

Answer 2

I think the combined mass was less afterward than the original mass due to escape of hydrogen gas in the atmosphere. According to the law of conservation of mass, mass is conserved and does not change in a chemical reaction which occurs in a closed system. However in an open system the mass may not be conserved and may change due to release of gases to the atmosphere. Like in this case, hydrogen gas produced in the reaction was released to the atmosphere thus reducing the final mass.